A close second is "My Body is a Cage," seeing all that Adam went through and how difficult it is for a transgender person.
Another contender is "Time Stands Still" and the aftermath of the shooting. Seeing different people's reactions to the shooting is heartbreaking, and it's set to Melissa McClelland's Rooftop. By the time it gets to the final shot of Emma and Toby, it's pretty difficult to remain dry-eyed.
After Alli loses her virginity, and they show her sitting alone in her ridiculously girly kiddy bedroom, being kissed goodnight by her mom, and making it all too clear that she was not ready for this.
When Sean's about to be shipped off to Afghanistan, he breaks down in front of Snake and admits that he's scared of what could happen to him while in combat.
Liberty giving up her baby.
Clare being sexually harassed by Asher.
Rick being bullied.
KC leaving at the end of "Rusty Cage".
Campbell's suicide and the characters' reactions to it in the episode "Bittersweet Symphony"
Sean breaking down in front of his parents because he killed Rick.
Craig's abuse storyline in "When Doves Cry," and then its conclusion in "Tears Are Not Enough"
Maya finally breaking down after Campbell's suicide towards the end of "Zombie"
Darcy realizing she was raped and the fallout from it, especially in "Live to Tell"
KC and Jenna deciding to give up their son for adoption in "Extraordinary Machine".
Imogen finding her dog... after he's been accidentally run over by her dad's car.
Drew's reaction to finding out about Adam's death. He says Adam's name and collapses into his mom's arms sobbing.
Dallas's reaction is just as bad. He gives a kind of traumatized stare and just backs away. Especially sad when you realize that this is the second time a friend of his has died, meaning he is probably remembering Cam's death all over again.
Becky, Drew, and Clare all have extremely heartbreaking moments in the next episode. Drew is completely torn up about his death, blaming himself for Adam's death and falling apart completely in front of Bianca at one point. Clare's cancer goes into remission, but she feels guilty about going to Adam's memorial because she got to live and he didn't. Becky tries to throw a bonfire in honor of Adam, but believes that all of his friends absolutely hate her and tries so hard to make it perfect. Of course, a large amount of people show up, leading to the most devastating speech that she gives while in tears.
Becky:So many people came. Just not the one I wanted... I'm so lost without you. Bye Adam Torres... I love you.